1. Field of the Invention
The present invention relates to electromagnetic methods and systems for detecting and/or locating individual people.
A particularly advantageous application lies in searching for avalanche victims.
Naturally other applications can be contemplated for the invention.
In particular, the invention applies to any detection and/or locating of a person hidden by an opaque obstacle and could for instance also be used when searching for people buried under rubble.
2. Description of the Related Art
Numerous systems for detecting and locating victims buried under avalanche debris are already known.
In particular, it has already been proposed to equip skiers with chemical beacons enabling them to be scented by search dogs. In this respect, reference can be made to French patent application FR 2 677 890.
Systems implementing electromagnetic transmission and reception have also been proposed.
Particular mention can be made of the ARVA system which is in particularly widely used in France (ARVA is a French acronym for assistance in searching for avalanche victims). In that system, skiers carry locating beacons that transmit at a frequency of 457 kHz.
Systems are also known such as the system sold in France under the name RECO, in which the garments of skiers are fitted with transmission dipoles designed, when subjected to electromagnetic illumination, to transmit at around 1.8 GHz.
Those various detection systems suffer from the major drawback of being capable of operating only if the person for whom a search is being made is provided with a beacon or a special transmission device; under such circumstances, a person is said to constitute a "cooperative" target.
Unfortunately, it still happens frequently that the people who have been buried are not so equipped, either because of their own imprudence, or because of poor evaluation of avalanche risk.
In addition, with a beacon, it is necessary for the beacon to be activated by the person concerned before becoming trapped in an avalanche.
An object of the invention is to offset those drawbacks.
A device for detecting the presence of a person hidden by an obstacle by transmitting and receiving microwaves is already known from the following publication:
"Microprocessor-controlled automatic clutter-cancellation circuits for microwave systems to sense physiological movements remotely through rubble", H. R. Chuang, et al., Feb. 13, 1990, San Jose, Feb. 13-15, 1990, pp. 177-181, IEEE.
Another device of that type has also been proposed in U.S. Pat. No. 5,448,501.
Nevertheless, both of those publications propose transmission and reception of monostatic type, i.e. using the same antenna.
Unfortunately, one of the main difficulties in detecting the internal movements of a person (breathing, heart beats) by transmitting and receiving microwaves is the very large energy ratio between the fixed echo due to the environment around the person, i.e. the "clutter", and the modulation induced by that person's own movements. Typically, this ratio is of the order of 100 dB. Also, a contribution to the clutter which is at least equal to if not greater than the signal that is to be detected comes from antenna matching which reinjects a fraction of transmitted power into the reception system (at best 1/10th of the transmitted power).
As a result, with monostatic systems, received signals cannot be processed directly since the clutter would otherwise be heavily saturating, so it is necessary to implement techniques for eliminating the clutter, e.g. by neutrodyning.
This gives rise to receiver electronics that are particularly complex.